This invention relates to a method of driving sheet piles into a rock substratum, both for the purpose of producing a sheet pile wall and also for subsequently anchoring the sheet pile wall. The invention is particularly intended for use in marine and river works, but is also suitable for every other civil engineering branch in which a sheet pile wall must be placed in a rock substratum.
In the engineering of inland and coastal ports or other waterways, provision must frequently be made for great differences in water level and, because of the trend towards ever larger ships, for very large draft. For the berths, sheet pile walls are generally used, which are composed of individual sheet piles adjoining one another and must be driven to a predetermined depth to provide a secure fixing. Frequently, however, this required driving depth cannot be achieved, because a rock substratum is present at quite shallow depth in the soil structure, preventing any driving beyond this depth.
Such a driving depth limited by a rock substratum is not harmful if the lower ends of the sheet piles can be fixed in the rock substratum sufficiently for the piles to obtain adequate fixity even though the theoretically calculated driving depth based upon soft soil conditions is not reached. Because the piles have sharp bottom edges, the bottom end of the piles can usually be driven without great difficulty to a sufficient penetration for reliable fixity, provided the rock substratum is comparatively soft. With a harder rock substratum, however, this simple driving is no longer possible, because the bottom ends of the piles become upset when driving is attempted, or they buckle sideways.
For these difficult ground conditions a process is known, in which the sheet piles are no longer driven, but are set into a channel blasted in the rock and concreted in there using underwater concrete. This process is extraordinarily complicated and costly, since before the generally V-shaped channel can be blasted, all loose rubble and the like overlying the rock must be removed, it being necessary in fairly loose overburden to maintain an angle of slope of 3:1, in order reliably to prevent the channel from filling up after blasting. After placing of the sheet piles in the V-trench and the subsequent concreting operation, it is frequently necessary to backfill the loose material that has been removed with so much trouble, in order to assure the final stability of the finished sheet pile wall. Apart from the extra work required, the real advantages of a sheet pile wall, namely its firm fixing in the soil resulting from the compaction of the substratum during driving, are completely lost, and instead the sheet pile wall is installed after the manner of a freestanding wall.
When a non-drivable rock substratum is present, difficulties are encountered not only in the construction of the sheet pile wall, but also corresponding difficulties exist in its subsequent lateral anchoring. After the erection of a sheet pile wall, that is after the driving of the individual sheet piles, it can be necessary for the upper end of the sheet pile wall to be secured against displacement, which is usually effected by means of anchors, which extend approximately at 45.degree. obliquely downwards from the upper edge of the sheet pile wall. Such an anchorage is provided especially for those sheet pile walls that are subjected to high soil pressure on one side, and which are supported in the driven state at the lower end only, as a consequence of the existing soil structure. Very frequently, these conditions occur in the securing of banks and in the construction of quay structures for waterways where a rock substratum is present at the place where land meets water.
The usual method of installing the anchors on the landward side of the sheet pile wall for a non-drivable rock substratum consists in drilling boreholes obliquely downwards from the seaward side from the upper edge of the wall, and then positioning simple anchors in the form of steel flats or angles loosely in the boreholes. The boreholes together with the anchors in them are then completely filled with concrete, which after hardening produces retaining forces predominantly due to frictional bond between the concrete and the wall of the borehole. Due to a lack of compaction of the ground material, such as is produced in driving, these retaining forces are not especially high per unit area, so that the frictional area between the concrete and the borehole wall must be designed to be correspondingly large. For this reason the anchors must frequently be made very long, resulting in a steep increase in the drilling costs.
A further disadvantage of this anchoring method lies in the fact that, if the bond friction is temporarily lost for example due to a blow or a shock, only sliding friction remains effective, which, as is well-known, provides much lower retaining forces than bonding friction. Such impacts can arise, for example, from careless berthing of a ship alongside the sheet pile wall or from other vibrations, for example from a road in the vicinity. Vibrations arising from blasting which must be carried out in the vicinity of the sheet pile wall are especially dangerous. This is always the case when the useful draft of the waterway adjacent to the wall must be increased and, on account of a rock substratum, blasting is unavoidable. There is then a risk that the sheet pile wall will lose the holding power of its anchors over a very large length immediately after blasting, and will be forced away by the soil pressure.
Considered overall, it has therefore hitherto not been possible, where a non-drivable rock substratum is present, to construct sheet pile walls by simple driving of the lower ends of the piles and to anchor them laterally where necessary by simple driving of anchors. Instead, it has hitherto been necessary to adopt other measures, in which no driving operations were involved and in which, apart from other disadvantges, it was also necessary to accept that the very high retaining forces which arise from driving piles due to the resultant compaction of the substratum could not be achieved.